这篇博文是参考别人的博客，结合源码自己又走了一遍，仅供自己记录学习。
我们要掌握android，那么关于android的View机制、动画原理这些都是必须要掌握的。不管是要了解View机制，还是android动画，我们应该都需要有Choreographer的知识，明白系统刷新机制到底是怎么样的，这样才能对其他方面有更好的辅助。我们就来学习一下Android中的Choreographer的运行机制。
我们都知道，应用层的一个Activity对应一个根View（也就是一个DecorView）、一个WindowState、一个ViewRootImpl，每个对象都非常重要，都是在Activity添加过程中重量级的对象，DecorView是当前Activity的根View，它里面管理着当前界面的View树；WindowState对象是当前Activity窗口在WindowManagerService中代理对象；ViewRootImpl则肩负着View的标准三步曲的处理和事件分发，而View绘制也是由Choreographer指导的，Choreographer的英文意思就是编舞者、舞蹈指挥，看着非常形象。那我们就从Choreographer对象的构建开始说起吧，它的构建是在ViewRootImpl的构造方法中的，代码如下
public ViewRootImpl(Context context, Display display) {
mContext = context;
mWindowSession = WindowManagerGlobal.getWindowSession();
mDisplay = display;
mBasePackageName = context.getBasePackageName();
mThread = Thread.currentThread();
mLocation = new WindowLeaked(null);
mLocation.fillInStackTrace();
mWidth = -1;
mHeight = -1;
mDirty = new Rect();
mTempRect = new Rect();
mVisRect = new Rect();
mWinFrame = new Rect();
mWindow = new W(this);
mTargetSdkVersion = context.getApplicationInfo().targetSdkVersion;
mViewVisibility = View.GONE;
mTransparentRegion = new Region();
mPreviousTransparentRegion = new Region();
mFirst = true; // true for the first time the view is added
mAdded = false;
mAttachInfo = new View.AttachInfo(mWindowSession, mWindow, display, this, mHandler, this,
context);
mAccessibilityManager = AccessibilityManager.getInstance(context);
mAccessibilityManager.addAccessibilityStateChangeListener(
mAccessibilityInteractionConnectionManager, mHandler);
mHighContrastTextManager = new HighContrastTextManager();
mAccessibilityManager.addHighTextContrastStateChangeListener(
mHighContrastTextManager, mHandler);
mViewConfiguration = ViewConfiguration.get(context);
mDensity = context.getResources().getDisplayMetrics().densityDpi;
mNoncompatDensity = context.getResources().getDisplayMetrics().noncompatDensityDpi;
mFallbackEventHandler = new PhoneFallbackEventHandler(context);
// 这里获取了Choreographer的实例
mChoreographer = Choreographer.getInstance();
mDisplayManager = (DisplayManager)context.getSystemService(Context.DISPLAY_SERVICE);
if (!sCompatibilityDone) {
sAlwaysAssignFocus = true;
sCompatibilityDone = true;
}
loadSystemProperties();
ViewDebugManager.getInstance().debugViewRootConstruct(mTag, context,
mThread, mChoreographer, mTraversalRunnable, this);
}
看一下Choreographer.getInstance()，获取的是一个单例。
public static Choreographer getInstance() {
return sThreadInstance.get();
}
sThreadInstance是一个ThreadLocal对象,ThreadLocal 是线程的局部变量， 是每一个线程所单独持有的，其他线程不能对其进行访问.
private static final ThreadLocal<Choreographer> sThreadInstance =
new ThreadLocal<Choreographer>() {
@Override
protected Choreographer initialValue() {
Looper looper = Looper.myLooper();
if (looper == null) {
throw new IllegalStateException("The current thread must have a looper!");
}
return new Choreographer(looper, VSYNC_SOURCE_APP);
}
};
为调用线程创建一个Choreographer实例，调用线程必须具备消息循环功能，因为ViewRootImpl对象的构造是在应用程序进程的UI主线程中执行的，因此创建的Choreographer对象将使用UI线程消息队列。
构造函数的第二个参数是VSYNC_SOURCE_APP,看一下它的定义，表示发出同步信号app
/**
* When retrieving vsync events, this specifies that the vsync event should happen at the normal
* vsync-app tick.
* <p>
* Needs to be kept in sync with frameworks/native/include/gui/ISurfaceComposer.h
*/
public static final int VSYNC_SOURCE_APP = 0;
private Choreographer(Looper looper, int vsyncSource) {
mLooper = looper;
//创建消息处理Handler
mHandler = new FrameHandler(looper);
//如果系统使用了Vsync机制，则注册一个FrameDisplayEventReceiver接收器
mDisplayEventReceiver = USE_VSYNC
? new FrameDisplayEventReceiver(looper, vsyncSource)
: null;
mLastFrameTimeNanos = Long.MIN_VALUE;
//屏幕刷新周期
mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
//创建回调数组
mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
//初始化数组
for (int i = 0; i <= CALLBACK_LAST; i++) {
mCallbackQueues[i] = new CallbackQueue();
}
}
变量USE_VSYNC用于表示系统是否是用了Vsync同步机制，该值是通过读取系统属性debug.choreographer.vsync来获取的。如果系统使用了Vsync同步机制，则创建一个FrameDisplayEventReceiver对象用于请求并接收Vsync事件，最后Choreographer创建了一个大小为4的CallbackQueue队列数组，用于保存不同类型的Callback。
当界面需要重绘时，都会调用到ViewRootImp类的scheduleTraversals()方法，这里的实现也比较简单，代码如下：
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
if (ViewDebugManager.DEBUG_SCHEDULETRAVERSALS) {
Log.v(mTag, "scheduleTraversals: mTraversalBarrier = " + mTraversalBarrier
+ ",this = " + this, new Throwable("scheduleTraversals"));
}
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
mTraversalScheduled表示是否已经发起重绘，每次scheduleTraversals()方法调用之后，就会将它置为true，然后在下次调用doTraversal()又先将它置为false，然后调用mChoreographer.postCallback()添加一个Runnable，请注意，第一个参数是Choreographer.CALLBACK_TRAVERSAL，在Choreographer当前，添加的类型一共有三种，分别是：CALLBACK_INPUT、CALLBACK_ANIMATION、CALLBACK_TRAVERSAL，分别表示事件回调、动画回调、绘制回调。postCallback()方法是转而调用postCallbackDelayed()方法的，最后一个参数delayMillis传的是0，表示当前的重绘不需要延时.
添加回调过程
postCallback@Choreographer.java
public void postCallback(int callbackType, Runnable action, Object token) {
postCallbackDelayed(callbackType, action, token, 0);
}
public void postCallbackDelayed(int callbackType,
Runnable action, Object token, long delayMillis) {
if (action == null) {
throw new IllegalArgumentException("action must not be null");
}
if (callbackType < 0 || callbackType > CALLBACK_LAST) {
throw new IllegalArgumentException("callbackType is invalid");
}
postCallbackDelayedInternal(callbackType, action, token, delayMillis);
}
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
if (DEBUG_FRAMES) {
Log.d(TAG, "PostCallback: type=" + callbackType
+ ", action=" + action + ", token=" + token
+ ", delayMillis=" + delayMillis);
}
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
此处获取当前时间，然后加上要延迟的时间，作为当前Callback的时间点，以这个时间点作为标准，把Callback对象添加到mCallbackQueues[callbackType]队列当中，这块的逻辑和Looper、MessageQueue、Handler中添加Message的逻辑很相似，大家可以对比学习。然后判断dueTime <= now，也就是执行时间到会执行scheduleFrameLocked。否则执行else分支，往当前的队列中添加一个Message，那么通过Handler机制就会进行处理，此处的mHandler是一个FrameHandler对象，我们来看一下FrameHandler的代码：
private final class FrameHandler extends Handler {
public FrameHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_DO_FRAME:
doFrame(System.nanoTime(), 0);
break;
case MSG_DO_SCHEDULE_VSYNC:
doScheduleVsync();
break;
case MSG_DO_SCHEDULE_CALLBACK:
doScheduleCallback(msg.arg1);
break;
}
}
}
这里的message消息也比较简单，MSG_DO_FRAME指系统在没有使用Vsync机制的时候，使用异步消息来刷新屏幕，当然，大家一定要理解，此处的刷新其实只是刷新屏幕工作的很小一部分，只是回调ViewRootImpl方法中添加的Runnable对象，最终是调用根View的draw方法，让每个子View有把自己的图像元素填充到分配好的显存当中，而要完全显示，还有很多工作要作，最终是在SurfaceFlinger类中对所有窗口的View进行合成，然后渲染，最终post到FrameBuffer上，才能显示出来的；MSG_DO_SCHEDULE_VSYNC当然就是指系统使用Vsync来刷新了；MSG_DO_SCHEDULE_CALLBACK就是指添加Callback或者FrameCallback完成的消息了。好了，我们继续看MSG_DO_SCHEDULE_CALLBACK的消息处理，它是调用doScheduleCallback(msg.arg1)来进行处理的，msg.arg1是刚才添加消息时的类型。我们整个看一下handleMessage()方法的代码，发现非常简单，这也是一个非常好的习惯，我们平时的代码当中，也应该尽量这样实现，这样一眼就可以看出来这个方法所要作的事情，把具体的处理放到每个细节方法中去。我们来看一下doScheduleCallback()方法的实现
void doScheduleCallback(int callbackType) {
synchronized (mLock) {
if (!mFrameScheduled) {
final long now = SystemClock.uptimeMillis();
if (mCallbackQueues[callbackType].hasDueCallbacksLocked(now)) {
scheduleFrameLocked(now);
}
}
}
}
mFrameScheduled和ViewRootImpl的scheduleTraversals()方法中的变量mTraversalScheduled作用是一样的，也是判断当前是否正在执行添加，然后调用(mCallbackQueues[callbackType].hasDueCallbacksLocked(now))判断是否已处理过Callback事务，该方法的判断也很简单，(mHead != null && mHead.dueTime <= now)，如果当前队列头不为空，并且队列头元素的时间点小于当前的时间点，那就说明是之前添加的，则需要对它进行处理；相反，如果队列头为空或者添加的时间点大于当前的时间点，也就是要延迟处理，则不需要任何操作。条件符合的话，就调用scheduleFrameLocked(now)进一步处理，我们来看一下scheduleFrameLocked()方法的实现：
private void scheduleFrameLocked(long now) {
if (!mFrameScheduled) {
mFrameScheduled = true;
//检查是否使用了Vsync机制
if (USE_VSYNC) {
//如果当前线程具备消息循环，则直接请求VSync信号
if (isRunningOnLooperThreadLocked()) {
scheduleVsyncLocked();
} else {//如果当前线程不具备消息循环，则通过主线程请求VSync信号
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
msg.setAsynchronous(true);
mHandler.sendMessageAtFrontOfQueue(msg);
}
} else { //如果系统没有使用VSync机制，则使用异步消息延时执行屏幕刷新
final long nextFrameTime = Math.max(
mLastFrameTimeNanos / NANOS_PER_MS + sFrameDelay, now);
Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, nextFrameTime);
}
}
}
一开始就把mFrameScheduled赋值为true，表示事务开始执行了，那么上面doScheduleCallback()方法当中的代码此该就不会再执行了。在该函数中考虑了两种情况，一种是系统没有使用Vsync机制，在这种情况下，首先根据屏幕刷新频率计算下一次刷新时间，通过异步消息方式延时执行doFrame()函数实现屏幕刷新。如果系统使用了Vsync机制，并且当前线程具备消息循环，则直接请求Vsync信号，否则就通过主线程来请求Vsync信号。FrameDisplayEventReceiver对象用于请求并接收Vsync信号，当Vsync信号到来时，系统会自动调用其onVsync()函数，在该回调函数中执行doFrame()实现屏幕刷新。
20140630191939812.png
看到这里，是不是感觉逻辑有点多了，开始乱了，转来转去的，系统到底要干啥？呵呵，我们暂停下来梳理一下，系统做了这么多事情最终的目的就是在下一次Vsync信号到来的时候，将Choreographer当中的四个队列中的事务执行起来，这些事务是应用层ViewRootImpl在scheduleTraversals()方法中添加进去的，在Choreographer当中，我们要先将外边传进来的Callback放入队列，然后就要去请求Vsync信号，因为Vsync信号是定时产生的，你不请求，它就不会理你，当然你收不到回调，也就不知道啥时候通知ViewRootImpl执行View的measure、layout、draw了，这样说一下，大家清楚我们要干什么了吗？我第一次看Choreographer类的代码时候，看了半天，也是乱了，所以这里大概理一下。
当VSYNC信号到达时，Choreographer doFrame()函数被调用
void doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
if (!mFrameScheduled) {
return; // no work to do
}
long intendedFrameTimeNanos = frameTimeNanos;
//保存起始时间
startNanos = System.nanoTime();
//由于Vsync事件处理采用的是异步方式，因此这里计算消息发送与函数调用开始之间所花费的时间
final long jitterNanos = startNanos - frameTimeNanos;
if (jitterNanos >= mFrameIntervalNanos) {
//计算函数调用期间所错过的帧数
final long skippedFrames = jitterNanos / mFrameIntervalNanos;
if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main thread.");
}
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
frameTimeNanos = startNanos - lastFrameOffset;
}
//如果frameTimeNanos小于一个屏幕刷新周期，则重新请求VSync信号
if (frameTimeNanos < mLastFrameTimeNanos) {
if (DEBUG_JANK) {
Log.d(TAG, "Frame time appears to be going backwards. May be due to a "
+ "previously skipped frame. Waiting for next vsync.");
}
scheduleVsyncLocked();
return;
}
//Contains information about the current frame for jank-tracking,
//mainly timings of key events along with a bit of metadata about
//view tree state
mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
mFrameScheduled = false;
mLastFrameTimeNanos = frameTimeNanos;
}
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
AnimationUtils.lockAnimationClock(frameTimeNanos / TimeUtils.NANOS_PER_MS);
//分别回调CALLBACK_INPUT、CALLBACK_ANIMATION、CALLBACK_TRAVERSAL、CALLBACK_COMMIT事件，这里是四个队列，8.1上新增了CALLBACK_COMMIT
mFrameInfo.markInputHandlingStart();
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
mFrameInfo.markAnimationsStart();
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
mFrameInfo.markPerformTraversalsStart();
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
} finally {
AnimationUtils.unlockAnimationClock();
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
/// M: PerfFrameInfo Mechanism
mFrameInfo.markDoFrameEnd();
if (DEBUG_FRAMES) {
final long endNanos = System.nanoTime();
Log.d(TAG, "Frame " + frame + ": Finished, took "
+ (endNanos - startNanos) * 0.000001f + " ms, latency "
+ (startNanos - frameTimeNanos) * 0.000001f + " ms.");
}
}
可以看到，平时我们通过Systrace进行性能分析的时候doFrame的Log就是在这里打出来的
20140630192141906.png
Choreographer类中分别定义了CallbackRecord、CallbackQueue内部类，CallbackQueue是一个按时间先后顺序保存CallbackRecord的单向循环链表。
20140630192252750.png
在Choreographer中定义了四个CallbackQueue队列，用数组mCallbackQueues表示，用于分别保存CALLBACK_INPUT、CALLBACK_ANIMATION、CALLBACK_TRAVERSAL、CALLBACK_COMMIT这四种类型的Callback，当调用Choreographer类的postCallback()函数时，就是往指定类型的CallbackQueue队列中通过addCallbackLocked()函数添加一个CallbackRecord项：首先构造一个CallbackRecord对象，然后按时间先后顺序插入到CallbackQueue链表中。从代码注释中，我们可以知道CALLBACK_INPUT是指输入回调，该回调优先级最高，首先得到执行，而CALLBACK_TRAVERSAL是指处理布局和绘图的回调，只有在所有异步消息都执行完后才得到执行，CALLBACK_ANIMATION是指动画回调，比CALLBACK_TRAVERSAL优先执行，CALLBACK_COMMIT是绘制完成后的一些操作，在CALLBACK_TRAVERSAL之后执行。输入事件放在第一，也是为了能尽快响应用户的操作，但是即使这样，Android的流畅性还是不如IOS从doFrame()函数中的doCallbacks调用就能印证这点。
当Vsync事件到来时，顺序执行CALLBACK_INPUT、CALLBACK_ANIMATION、CALLBACK_TRAVERSAL和CALLBACK_COMMIT对应CallbackQueue队列中注册的回调。
void doCallbacks(int callbackType, long frameTimeNanos) {
CallbackRecord callbacks;
synchronized (mLock) {
final long now = System.nanoTime();
//从指定类型的CallbackQueue队列中查找执行时间到的CallbackRecord
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
now / TimeUtils.NANOS_PER_MS);
if (callbacks == null) {
return;
}
mCallbacksRunning = true;
// Update the frame time if necessary when committing the frame.
// We only update the frame time if we are more than 2 frames late reaching
// the commit phase. This ensures that the frame time which is observed by the
// callbacks will always increase from one frame to the next and never repeat.
// We never want the next frame's starting frame time to end up being less than
// or equal to the previous frame's commit frame time. Keep in mind that the
// next frame has most likely already been scheduled by now so we play it
// safe by ensuring the commit time is always at least one frame behind.
if (callbackType == Choreographer.CALLBACK_COMMIT) {
final long jitterNanos = now - frameTimeNanos;
Trace.traceCounter(Trace.TRACE_TAG_VIEW, "jitterNanos", (int) jitterNanos);
if (jitterNanos >= 2 * mFrameIntervalNanos) {
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos
+ mFrameIntervalNanos;
if (DEBUG_JANK) {
Log.d(TAG, "Commit callback delayed by " + (jitterNanos * 0.000001f)
+ " ms which is more than twice the frame interval of "
+ (mFrameIntervalNanos * 0.000001f) + " ms! "
+ "Setting frame time to " + (lastFrameOffset * 0.000001f)
+ " ms in the past.");
mDebugPrintNextFrameTimeDelta = true;
}
frameTimeNanos = now - lastFrameOffset;
mLastFrameTimeNanos = frameTimeNanos;
}
}
}
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
//由于CallbackQueues是按时间先后顺序排序的，因此遍历执行所有时间到的CallbackRecor
for (CallbackRecord c = callbacks; c != null; c = c.next) {
if (DEBUG_FRAMES) {
Log.d(TAG, "RunCallback: type=" + callbackType
+ ", action=" + c.action + ", token=" + c.token
+ ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
}
c.run(frameTimeNanos);
}
} finally {
synchronized (mLock) {
mCallbacksRunning = false;
do {
final CallbackRecord next = callbacks.next;
recycleCallbackLocked(callbacks);
callbacks = next;
} while (callbacks != null);
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
该类就是按时间顺序先后执行到时的CallbackRecord
private static final class CallbackRecord {
public CallbackRecord next;
public long dueTime;
public Object action; // Runnable or FrameCallback
public Object token;
public void run(long frameTimeNanos) {
if (token == FRAME_CALLBACK_TOKEN) {
((FrameCallback)action).doFrame(frameTimeNanos);
} else {
((Runnable)action).run();
}
}
}
我们知道Choreographer对外提供了两个接口函数用于注册指定的Callback，postCallback()用于注册Runnable对象，而postFrameCallback()函数用于注册FrameCallback对象，无论注册的是Runnable对象还是FrameCallback对象，在CallbackRecord对象中统一装箱为Object类型。在执行其回调函数时，就需要区别这两种对象类型，如果注册的是Runnable对象，则调用其run()函数，如果注册的是FrameCallback对象，则调用它的doFrame()函数。
20140630192623921.png
Vsync请求过程
我们知道在Choreographer构造函数中，构造了一个FrameDisplayEventReceiver对象，用于请求并接收Vsync信号
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
implements Runnable {
private boolean mHavePendingVsync;
private long mTimestampNanos;
private int mFrame;
public FrameDisplayEventReceiver(Looper looper, int vsyncSource) {
super(looper, vsyncSource);
}
@Override
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
// Ignore vsync from secondary display.
// This can be problematic because the call to scheduleVsync() is a one-shot.
// We need to ensure that we will still receive the vsync from the primary
// display which is the one we really care about. Ideally we should schedule
// vsync for a particular display.
// At this time Surface Flinger won't send us vsyncs for secondary displays
// but that could change in the future so let's log a message to help us remember
// that we need to fix this.
if (builtInDisplayId != SurfaceControl.BUILT_IN_DISPLAY_ID_MAIN) {
Log.d(TAG, "Received vsync from secondary display, but we don't support "
+ "this case yet. Choreographer needs a way to explicitly request "
+ "vsync for a specific display to ensure it doesn't lose track "
+ "of its scheduled vsync.");
scheduleVsync();
return;
}
// Post the vsync event to the Handler.
// The idea is to prevent incoming vsync events from completely starving
// the message queue. If there are no messages in the queue with timestamps
// earlier than the frame time, then the vsync event will be processed immediately.
// Otherwise, messages that predate the vsync event will be handled first.
long now = System.nanoTime();
if (timestampNanos > now) {
Log.w(TAG, "Frame time is " + ((timestampNanos - now) * 0.000001f)
+ " ms in the future! Check that graphics HAL is generating vsync "
+ "timestamps using the correct timebase.");
timestampNanos = now;
}
if (mHavePendingVsync) {
Log.w(TAG, "Already have a pending vsync event. There should only be "
+ "one at a time.");
} else {
mHavePendingVsync = true;
}
mTimestampNanos = timestampNanos;
mFrame = frame;
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
}
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame);
}
}
我们可以看到这里的mTimestampNanos时间定义都是纳秒级别的，因为Vsync信号是用来同步屏幕刷新频率的，所以对时间的要求非常高，才采用了纳秒级别的，mDisplayEventReceiver类变量是在Choreographer的构造方法中赋值的，我们继续来看它的scheduleVsync()方法的实现，因为FrameDisplayEventReceiver类是继承DisplayEventReceiver的，而它没用对scheduleVsync()方法重写，所以是调用父类的
private void scheduleVsyncLocked() {
//申请Vsync信号
mDisplayEventReceiver.scheduleVsync();
}
FrameDisplayEventReceiver继承于DisplayEventReceiver类，Vsync请求在DisplayEventReceiver中实现。
public void scheduleVsync() {
if (mReceiverPtr == 0) {
Log.w(TAG, "Attempted to schedule a vertical sync pulse but the display event "
+ "receiver has already been disposed.");
} else {
nativeScheduleVsync(mReceiverPtr);
}
}
它的实现很简单，判断描述符mReceiverPtr是否合法，如果非法就打印日志，什么也不作了，合法的话，就继续调用native方法nativeScheduleVsync(mReceiverPtr)来请求Vsync信号。nativeScheduleVsync()方法实现在android_view_DisplayEventReceiver.cpp当中，是通过定义JNINativeMethod gMethods[]来定义方法调用指针的，因为此类的代码不多，这里就全部贴出来，方便大家查看：
namespace android {
static struct {
jclass clazz;
jmethodID dispatchVsync;
jmethodID dispatchHotplug;
} gDisplayEventReceiverClassInfo;
class NativeDisplayEventReceiver : public DisplayEventDispatcher {
public:
NativeDisplayEventReceiver(JNIEnv* env,
jobject receiverWeak, const sp<MessageQueue>& messageQueue, jint vsyncSource);
void dispose();
protected:
virtual ~NativeDisplayEventReceiver();
private:
jobject mReceiverWeakGlobal;
sp<MessageQueue> mMessageQueue;
DisplayEventReceiver mReceiver;
virtual void dispatchVsync(nsecs_t timestamp, int32_t id, uint32_t count);
virtual void dispatchHotplug(nsecs_t timestamp, int32_t id, bool connected);
};
NativeDisplayEventReceiver::NativeDisplayEventReceiver(JNIEnv* env,
jobject receiverWeak, const sp<MessageQueue>& messageQueue, jint vsyncSource) :
DisplayEventDispatcher(messageQueue->getLooper(),
static_cast<ISurfaceComposer::VsyncSource>(vsyncSource)),
mReceiverWeakGlobal(env->NewGlobalRef(receiverWeak)),
mMessageQueue(messageQueue) {
ALOGV("receiver %p ~ Initializing display event receiver.", this);
}
NativeDisplayEventReceiver::~NativeDisplayEventReceiver() {
JNIEnv* env = AndroidRuntime::getJNIEnv();
env->DeleteGlobalRef(mReceiverWeakGlobal);
ALOGV("receiver %p ~ dtor display event receiver.", this);
}
void NativeDisplayEventReceiver::dispose() {
ALOGV("receiver %p ~ Disposing display event receiver.", this);
DisplayEventDispatcher::dispose();
}
void NativeDisplayEventReceiver::dispatchVsync(nsecs_t timestamp, int32_t id, uint32_t count) {
JNIEnv* env = AndroidRuntime::getJNIEnv();
ScopedLocalRef<jobject> receiverObj(env, jniGetReferent(env, mReceiverWeakGlobal));
if (receiverObj.get()) {
ALOGV("receiver %p ~ Invoking vsync handler.", this);
env->CallVoidMethod(receiverObj.get(),
gDisplayEventReceiverClassInfo.dispatchVsync, timestamp, id, count);
ALOGV("receiver %p ~ Returned from vsync handler.", this);
}
mMessageQueue->raiseAndClearException(env, "dispatchVsync");
}
void NativeDisplayEventReceiver::dispatchHotplug(nsecs_t timestamp, int32_t id, bool connected) {
JNIEnv* env = AndroidRuntime::getJNIEnv();
ScopedLocalRef<jobject> receiverObj(env, jniGetReferent(env, mReceiverWeakGlobal));
if (receiverObj.get()) {
ALOGV("receiver %p ~ Invoking hotplug handler.", this);
env->CallVoidMethod(receiverObj.get(),
gDisplayEventReceiverClassInfo.dispatchHotplug, timestamp, id, connected);
ALOGV("receiver %p ~ Returned from hotplug handler.", this);
}
mMessageQueue->raiseAndClearException(env, "dispatchHotplug");
}
static jlong nativeInit(JNIEnv* env, jclass clazz, jobject receiverWeak,
jobject messageQueueObj, jint vsyncSource) {
sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, messageQueueObj);
if (messageQueue == NULL) {
jniThrowRuntimeException(env, "MessageQueue is not initialized.");
return 0;
}
sp<NativeDisplayEventReceiver> receiver = new NativeDisplayEventReceiver(env,
receiverWeak, messageQueue, vsyncSource);
status_t status = receiver->initialize();
if (status) {
String8 message;
message.appendFormat("Failed to initialize display event receiver. status=%d", status);
jniThrowRuntimeException(env, message.string());
return 0;
}
receiver->incStrong(gDisplayEventReceiverClassInfo.clazz); // retain a reference for the object
return reinterpret_cast<jlong>(receiver.get());
}
static void nativeDispose(JNIEnv* env, jclass clazz, jlong receiverPtr) {
NativeDisplayEventReceiver* receiver =
reinterpret_cast<NativeDisplayEventReceiver*>(receiverPtr);
receiver->dispose();
receiver->decStrong(gDisplayEventReceiverClassInfo.clazz); // drop reference held by the object
}
static void nativeScheduleVsync(JNIEnv* env, jclass clazz, jlong receiverPtr) {
sp<NativeDisplayEventReceiver> receiver =
reinterpret_cast<NativeDisplayEventReceiver*>(receiverPtr);
status_t status = receiver->scheduleVsync();
if (status) {
String8 message;
message.appendFormat("Failed to schedule next vertical sync pulse. status=%d", status);
jniThrowRuntimeException(env, message.string());
}
}
static const JNINativeMethod gMethods[] = {
/* name, signature, funcPtr */
{ "nativeInit",
"(Ljava/lang/ref/WeakReference;Landroid/os/MessageQueue;I)J",
(void*)nativeInit },
{ "nativeDispose",
"(J)V",
(void*)nativeDispose },
// @FastNative
{ "nativeScheduleVsync", "(J)V",
(void*)nativeScheduleVsync }
};
int register_android_view_DisplayEventReceiver(JNIEnv* env) {
int res = RegisterMethodsOrDie(env, "android/view/DisplayEventReceiver", gMethods,
NELEM(gMethods));
jclass clazz = FindClassOrDie(env, "android/view/DisplayEventReceiver");
gDisplayEventReceiverClassInfo.clazz = MakeGlobalRefOrDie(env, clazz);
gDisplayEventReceiverClassInfo.dispatchVsync = GetMethodIDOrDie(env,
gDisplayEventReceiverClassInfo.clazz, "dispatchVsync", "(JII)V");
gDisplayEventReceiverClassInfo.dispatchHotplug = GetMethodIDOrDie(env,
gDisplayEventReceiverClassInfo.clazz, "dispatchHotplug", "(JIZ)V");
return res;
}
} // namespace android
我们来看一下nativeScheduleVsync方法的定义，{ "nativeScheduleVsync", "(J)V", (void*)nativeScheduleVsync }，这里需要说明一下，java方法和JNI方法存在着对应关系，"(J)V"括号里边的表示该方法的入参，括号外边的表示返回值J表示long，而返回值V表示Void。好了，我们继续看这个方法的实现，它将java层传进来的描述符强制转换为NativeDisplayEventReceiver对象，这样的处理在JNI当中是非常多见的，大家要熟悉。然后调用它的scheduleVsync()方法，最后根据返回值判断当前请求Vsync信号是否成功，如果status非0，则抛出RuntimeException异常。很明显，我们从这都可以猜出，正常情况下，返回的status应该为0了。
我们继续来看NativeDisplayEventReceiver::scheduleVsync()方法的处理逻辑。
status_t DisplayEventDispatcher::scheduleVsync() {
if (!mWaitingForVsync) {
ALOGV("dispatcher %p ~ Scheduling vsync.", this);
// Drain all pending events.
nsecs_t vsyncTimestamp;
int32_t vsyncDisplayId;
uint32_t vsyncCount;
if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount)) {
ALOGE("dispatcher %p ~ last event processed while scheduling was for %" PRId64 "",
this, ns2ms(static_cast<nsecs_t>(vsyncTimestamp)));
}
status_t status = mReceiver.requestNextVsync();
if (status) {
ALOGW("Failed to request next vsync, status=%d", status);
return status;
}
mWaitingForVsync = true;
}
return OK;
}
首先检查mWaitingForVsync，如果当前正在请求Vsync信号，则就不需要重复请求了，只有在当前未请求的时候，才需要发出新的请求，然后调用processPendingEvents()将当前队列中还存在receiver处理掉.
bool DisplayEventDispatcher::processPendingEvents(
nsecs_t* outTimestamp, int32_t* outId, uint32_t* outCount) {
bool gotVsync = false;
DisplayEventReceiver::Event buf[EVENT_BUFFER_SIZE];
ssize_t n;
while ((n = mReceiver.getEvents(buf, EVENT_BUFFER_SIZE)) > 0) {
ALOGV("dispatcher %p ~ Read %d events.", this, int(n));
for (ssize_t i = 0; i < n; i++) {
const DisplayEventReceiver::Event& ev = buf[i];
switch (ev.header.type) {
case DisplayEventReceiver::DISPLAY_EVENT_VSYNC:
// Later vsync events will just overwrite the info from earlier
// ones. That's fine, we only care about the most recent.
gotVsync = true;
*outTimestamp = ev.header.timestamp;
*outId = ev.header.id;
*outCount = ev.vsync.count;
break;
case DisplayEventReceiver::DISPLAY_EVENT_HOTPLUG:
dispatchHotplug(ev.header.timestamp, ev.header.id, ev.hotplug.connected);
break;
default:
ALOGW("dispatcher %p ~ ignoring unknown event type %#x", this, ev.header.type);
break;
}
}
}
if (n < 0) {
ALOGW("Failed to get events from display event dispatcher, status=%d", status_t(n));
}
return gotVsync;
}
因此方法与我们的流程不相关，这里就不展开了，大致是使用pipe机制将mReceiver中还存在的receiver一一读出，大家如果了解Linux机制的话，就知道pipe机制对应了两个管道，管道中的数据被读出之后，也就相应的从管道中移除了，所以不需要两端对数据做任何移除的处理，每一个receiver处理完成后，就设置一下gotVsync = true，
outTimestamp = ev.header.timestamp，outId = ev.header.id，*outCount = ev.vsync.count，gotVsync的意思就是当前的receiver已经收到Vsync信号通知了。好了，我们回到主流程，scheduleVsync()方法当中处理完队列中的receiver后，就开始调用mReceiver.requestNextVsync()请求新的Vsync信号了，mReceiver是一个DisplayEventReceiver对象，我们来看一下requestNextVsync()方法的实现.
status_t DisplayEventReceiver::requestNextVsync() {
if (mEventConnection != NULL) {
mEventConnection->requestNextVsync();
return NO_ERROR;
}
return NO_INIT;
}
requestNextVsync()方法中直接调用mEventConnection->requestNextVsync()来请求Vsync信号，mEventConnection对象是在DisplayEventReceiver类的构造函数中创建的，mEventConnection = sf->createDisplayEventConnection()，sf就是SurfaceFlinger对象，SurfaceFlinger类的createDisplayEventConnection()实现也非常简单，就是调用mEventThread->createEventConnection()
sp<IDisplayEventConnection> SurfaceFlinger::createDisplayEventConnection(
ISurfaceComposer::VsyncSource vsyncSource) {
if (vsyncSource == eVsyncSourceSurfaceFlinger) {
return mSFEventThread->createEventConnection();
} else {
return mEventThread->createEventConnection();
}
}
EventThread一直在无限循环threadLoop()中请求Vsync信号的，当收到一个Vsync信号后，会调用status_t err = conn->postEvent(event)来进行分发，conn也就是上面的EventThread::Connection对象了，最后经过处理，回调到NativeDisplayEventReceiver::handleEvent(int receiveFd, int events, void* data)方法当中，这里同样processPendingEvents()处理完队列中的回调后，就调用dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount)开始分发了，在NativeDisplayEventReceiver::dispatchVsync()这个方法中是通过当前的native层的执行环境env回调到java层的，env->CallVoidMethod(mReceiverObjGlobal,
gDisplayEventReceiverClassInfo.dispatchVsync, timestamp, id, count)，再往下就回调到java层中DisplayEventReceiver类的dispatchVsync()方法中了。它里边的实现就是调用onVsync()，而FrameDisplayEventReceiver复写了onVsync()方法，所以就执行到Choreographer.FrameDisplayEventReceiver中的onVsync()方法了。
onVsync()方法中以this为对象，向mHandler中添加了一个消息，消息处理的时候，就会调用它的run()方法了。run方法中直接调用doFrame()来进行处理。这个方法具体已经在上面分析过了。下面通过图来梳理Vsync请求的调用流程：
20140630193116703.png
这里就是将每种类型的事件队列中的元素取出来，通过for循环一一调用他们的run()方法了，调用完成后，将队列中的Callback回收掉。而这里的CallbackRecord对象就是我们在ViewRootImpl类当中添加的InvalidateOnAnimationRunnable、mConsumedBatchedInputRunnable、mTraversalRunnable这三类对象了，那么回到View的流程中，收到Vsync信号后，就会回调mTraversalRunnable的run()方法，再次发起一次measure、layout、draw流程，那么也就和Vsync信号对接上了。
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